This invention relates to devices known as stents which provide support to a vessel, such as a blood vessel, and in particular to stents that are both temporary and removable.
Obstructive coronary artery disease is one of most serious health problems facing our society today. This disease is the result of the deposit of fatty substances on the interior surface of the walls of the arteries. The build up or lesion of such deposits results in a narrowing of the diameter of the artery which restricts the blood flow through the artery. This condition wherein the artery is narrowed is known as stenosis. The lesion may form in any part of the artery and in some instances the deposits may form at the intersection between two arteries, that is, where the section where the two arteries form a generally "Y" configuration (e.g. bifurcate, trifurcate, and so on).
There have been significant developments of the treatment of such obstructive coronary disease in the recent past. Coronary artery bypass graft surgery is often used to treat this disease. Bypass surgery, however, has the disadvantage that it is extremely invasive and traumatic to the patient. Some of the recent developments provide a less invasive and less traumatic alternative to bypass surgery. Two of these recent developments are known as angioplasty and atherectomy procedures.
Angioplasty is a procedure in which a balloon is positioned on the inside of the artery at the site of the lesion and expanded in order to compress the materials at the lesion and thus open the restricted area in the artery. In this procedure, an elastic balloon is attached to one end of a small diameter flexible catheter which includes means for inflating the balloon from the other end of the catheter. The catheter is maneuvered through the patient's vessels to the site of the lesion with the balloon in uninflated form. When the uninflated balloon is properly positioned at the lesion, the balloon is then inflated to dilatate the restricted area.
Atherectomy is a procedure in which a small cutting tool is attached to the end of a small diameter flexible catheter and maneuvered through the patient's arterial system to the site of the lesion in the diseased artery. When the cutting tool is properly positioned, the tool is used to cut and remove the deposits from the surface of the diseased artery.
Although these two procedures provide less traumatic alternatives to bypass surgery, they are not without risk. It is possible that following procedures such as angioplasty or atherectomy the artery or blood vessel may collapse or be susceptible to constriction. In some instances it may also be necessary to abort or "bail-out" procedures such as angioplasty or atherectomy due to some type of unexpected complication.
In these situations it is necessary to maintain the integrity of the region of the artery until the artery is repaired or stabilizes. That is, following some angioplasty or atherectomy procedures or in a "bail-out" situation, it may be necessary to provide support to a artery or blood vessel on a temporary basis while there is an immediate risk that the region may collapse. This support must be provided until the region is repaired or stabilized. To provide this support, a device known as a stent may be installed at the desired region. A stent is a device which is used to hold or maintain the diameter of the artery or vessel.
Although some stents are available in the art, these are generally of the type intended for permanent use. This type of permanent stent is implanted in a patient's vascular system and remains in place after the procedure or operation. Such permanent types of stents are shown, for example, in U.S. Pat. Nos. 4,913,141, 4,878,906, 4,856,516 and 4,820,298. These permanent type of stents may not always be desired for the situations described above. First, it may be unnecessary and even undesirable to install a permanent device when only temporary support is needed. Further, these permanent type of stents may require a relatively complicated procedure to install. Further, use of permanent stents results in extended hospital observation and recovery time. Additionally, a complement of drug therapies are required in order to offset the bioreaction resulting in thrombus formation or smooth muscle cell proliferation on the stent surface. These drug therapies may be required for a significant period of time until new normal endothelial cells have formed. In situations such as a "bail-out it is desirable for the physician to have the ability to quickly maneuver the stent to the desired location and quickly and easily place the stent in its operating mode.
A temporary stent on the other hand may be particularly useful in situations where it is intended to be used in the patient only for several minutes or days and then removed. For example, use of a temporary stent in a bail-out situation will enable the physician to defer a more complicated procedure until a patient's condition is more stable, or in some cases eliminate further procedures by resecuring the vessel geometry which allows near normal blood flow.
A temporary stent may have particular usefulness in situations such as when an intimal flap is encountered or during occurrrences of vasoconstriction or vasospasm or in situations in which there is a potential for such conditions to occur such as following angioplasty. An intimal flap occurs when a portion of the vessel wall partially or completely tears off and hangs down into the blood flow path. An intimal flap may occur during or after an angioplasty procedure. If the flap is large, it may entirely occlude the vessel lumen. The flap may heal itself if it can be maintained in place against the vessel from which it tore.
Vasoconstriction or vasospasm also may occur during or after angioplasty. Vasoconstriction or vasospasm in many cases may accompany the occurrence of an intimal flap, but in many other cases, vasoconstriction or vasospasm may occur independent of an intimal flap. During vasoconstriction or vasospasm, muscles around the vessel contract and can partially or completely occlude the vessel. If the vessel can be maintained open, vasoconstriction or vasospasm may cease after a period of time. Medicines may be administered to treat the vasoconstriction or vasospasm. Whereas a small force may be sufficient to maintain an intimal flap against the vessel wall and maintain blood flow, a significantly greater force would usually be needed to keep a vessel open during an episode of vasoconstriction or vasospasm.
It is essential that a temporary stent be relatively easy to both install and remove. Since the temporary stent remains in place for a period of time, it is important that the temporary stent not block the flow of blood through the vessel. That is, the blood must be able to travel through the vessel in which the temporary stent is installed while the stent is in place. Further, since the lesions often occur at the intersection of two vessels, in order to position the temporary stent it is may be necessary to place the stent across the intersection. Therefore, it is critical that the stent provide a flow path radially as well as axially or longitudinally. This arrangement will allow blood flow to both of the intersecting arteries.
It is also desirable to have the ability to deliver medicines to the vessel either upstream or downstream of temporary stent while the stent is in place.
Since the temporary stent will be removed after a period of time, it is important that the temporary stent not permanently adhere to the inner walls of the vessel in which it is placed. In addition, a temporary stent should have no tendency, or only a minimal tendency, to cause clotting.
Accordingly, it is an object of the present invention to provide a stent that may be placed temporarily in a patient's vascular system and which is readily removable.